Liquid detergents have hitherto been used mainly for light duty applications such as dish washing. The market for heavy duty detergents, e.g. laundry detergents, has been dominated by powders, due to the difficulty of getting an effective amount of surfactant and in particular of Builder into a stable liquid formulation. Such liquids should in theory be cheaper than powder detergents since they would avoid the need to dry and would in many instances replace the sulphate filler conventionally used in powder detergents with water. They also offer the possibilities of greater convenience and more rapid dissolution in wash water than powder. Attempts to provide solutions of the Functional Ingredients have been relatively unsuccessful commercially. One reason for this lack of success has been that the most commonly used and cost effective Functional Ingredients, e.g. sodium tripolyphosphate and sodium dedecyl benzene sulphonate, are insufficiently soluble in aqueous formulations. Potassium pyrophosphate and amine salts of the Active Ingredients which are more soluble, have been tried as alternatives but have not been found cost effective.
Unbuilt liquid detergents containing high levels of surfactant have been marketed for laundry use, but are unsuitable for hard water areas and have enjoyed only limited success.
A different approach is to attempt to suspend the excess Builder as a solid in the liquid solution of surfactant. The problem however has been to stabilise the system to maintain the Builder in suspension and prevent sedimentation. This has in the past required relatively sophisticated formulations, preventing realisation of the potential cost saving, and relatively low concentrations of solid Builder, giving limited washing effectiveness. This approach has been conditioned by certain assumptions: that the detergent should as far as possible be in solution; that the amount of suspended solid should be minimised to avoid difficulties in stabilising the suspension against sedimentation; and that special thickeners or stabilisers were essential to prevent sedimentation.
The products hitherto introduced commercially have suffered from certain serious drawbacks. In particular, the individual formulations have been proved highly sensitive to relatively small variations in composition and manufacturing procedure. Departure from a particular composition, optimised within fairly narrow limits, generally results in instability and diminished shelf life. The formulator has therefore been restricted to particular ingredients and proportions, which have not included many of the most effective combinations of surfactants and Builder for laundry purposes.
Because no general adequate theoretical explanation for the stability of such systems has been proposed, it has not proved possible to predict which formulations will be stable and which unstable, or how to set about stabilising any given surfactant Builder combination which may be desired for reasons of washing effectiveness or cost. Each formulation has had to be discovered by trial and error, and little flexibility has existed for adapting the individual formulations to special requirements.
Moreover, in general, the Payload has been undesirably low. In addition, the proportion of Builder to Active Ingredient has generally been less than is preferred for optimum washing, and expensive ingredients, not usually required in powder formulations, have often been needed to increase the amount of Functional Ingredient in solution, and to inhibit sedimentation of the suspended solid.